A. Field of the Invention
The present invention relates to antennas used to transmit and receive high frequency (HF) radio signals in the approximate range of 2 MHZ to 30 MHZ. More particularly, the invention relates to an HF monopole antenna which is attachable to a motor vehicle and which is rapidly tunable to maximize efficiency of transmitting and receiving radio signals at different selected frequencies contained in a relatively wide band.
B. Description of Background Art
Two-way radio communications between locations spaced apart at substantial distances, e.g., tens, hundreds or thousands of miles apart cannot utilize direct line-of-sight paths, because of the curvature of the earth's surface. Therefore, such radio communications use radio frequency signals which operate in a high frequency range, e.g., 2 MHZ to 30 MHZ, because radio signals in the HF range are reflected from the earth's ionosphere. Thus, HF radio signals can be transmitted obliquely upwards towards the ionosphere, and bounced back towards the earth beyond the visual horizon Signals which are reflected from the ionosphere and impinge the earth's surface can also be reflected back towards the ionosphere. In this way, multiple consecutive reflections of signals between the ground and the ionosphere can provide an effective means of transmitting HF signals over long distances. The ionosphere is electrically conductive and hence, effective in reflecting radio signals because of the presence of charged particles consisting of positively charged gas molecules and electrons which have been stripped from neutral molecules by impacting particles or energetic photons.
Because the ionized particles of atmospheric gases in the ionosphere are created largely by radiation from the sun, the concentration of ionized particles varies widely on a daily basis. As can be readily understood, the production of ionized particles overhead during daylight is greater than during nighttime. However, the re-combination of ions to form neutral atoms to thus decrease the concentration of ions depends on many variables, such as upper atmosphere winds. Moreover, in addition to diurnal variation in ion concentrations in the atmosphere, a variation in the sun's output of protons, which can be substantial, causes the ion concentration in the ionosphere to vary in unpredictable ways.
It is an observed and theoretically understand fact that the reflectivity of radio signals from the ionosphere depends both on the concentration of ions in the ionosphere, and upon the frequency of HF radio signals which are incident upon the ionosphere. Therefore, as is well know to HAM radio operators, as well as government agencies such as U.S. military services which communicate via HF radio signals, that it is often necessary to adjust the frequency of transmitted HF signals to values which are most effectively reflected from the ionosphere, to maximize the strength of a radio signal received at a distant location.
In addition to temporal variations of the reflectivity of the ionosphere which make adjustability of HF radio signal frequencies desirable there are spatial variations. Thus, for example, the optimum frequency for most effectively bouncing a transmitted signal from the ionosphere from a transmitter to a receiver station due North of the transmitter may differ from the optimum frequency for transmitting a signal to a receiver located West of the transmitter.
There are other reasons why it would be desirable to provide a HF communication link with frequency adjustability. For example, it is sometimes required that a fixed command and control site base station transmit different messages to different remote fixed or mobile receivers. Thus, by sending a sequence of messages, each at a different pre-selected frequency, a different message can be sent from a central command and control site to different intended recipients. Moreover, an operator at either a base station or a remote site can adjust the frequency of a transmitted radio signal and inquire of the distant recipient which frequencies provide the strongest received signals.
Also, it is possible to enhance the security of a radio frequency signal transmission by a technique known as frequency-hopping, in which information such as a voice communication message or a data stream is partitioned or time divided into a sequence of packets, each of which is sent sequentially at a different RF-carrier frequency.
In U.S. Pat. Nos. 6,275,195 and 6,496,154, the present inventor disclosed a frequency adjustable mobile monopole antenna that uses a circular shorting disk which is slidably moved by a motor-driven lead screw within the bore of a solenoidal loading coil disposed between a base mast and radiating whip section of a mobile monopole antenna. By moving the shorting disk up or down to contact and short-out a larger or smaller number of the coil turns, the inductance of the loading coil can be reduced or increased to cause the antenna to resonate at higher or lower selected frequencies. The disclosed frequency adjustable antenna has proven to be highly effective in performing its intended task of providing a frequency adjustable mobile antenna which can be reliably tuned by remote motor command signals to adjust the inductance of the loading coil to values which optimize transmission and reception efficiency over a wide band of frequencies. However, a need has remained for an adjustable frequency mobile monopole communication antenna which can be very rapidly tuned to thus meet greater speed-demanding applications such as frequency-hopping mentioned above. The present invention was motivated at least in part by this received need.